1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine capable of fuel combustion at lean air-fuel ratios (a lean burn engine) with an EGR (exhaust gas recirculation) system and with a zeolite catalyst for reducing NOx (a lean NOx catalyst) installed in an exhaust conduit of the engine.
2. Description of the Prior Art
It is known that a zeolite catalyst, when installed in an exhaust conduit of a lean burn engine, is effective to reduce nitrogen oxides (NOx) included in the exhaust gas of the engine when the engine is operated at lean air-fuel ratios. The zeolite catalyst needs hydrocarbons (HC), however, to reduce NOx under these oxidizing gas conditions, as disclosed in Japanese Patent Publication SHO 63-283727.
Though the exhaust gas itself includes HC, the amount of HC often is insufficient for the zeolite catalyst to effectively reduce NOx. To solve the problem of insufficient HC, Japanese Patent Publications HEI 1-186015 and HEI 2-51741 propose using exhaust gas recirculation (EGR).
More particularly, Patent Publication HEI 1-186015 discloses a procedure to execute EGR and to shift the air-fuel ratio toward the richer side within a lean air-fuel ratio range when the exhaust gas temperature is high and HC is directly oxidized, resulting in an insufficient HC condition. In this procedure, the EGR rate is increased at medium engine loads and medium engine speeds.
Patent Publication HEI 2-51741 discloses a procedure to execute EGR and to shift the air-fuel ratio toward the richer side within a lean air-fuel range during acceleration when a relatively large amount of HC is exhausted from the engine and therefore the HC amount is insufficient. In this procedure, the EGR rate (EGR amount) is increased when the exhaust gas temperature is high.
However, since the prior art only increases HC concentration of the exhaust gas, EGR is not effectively utilized for increasing a NOx purification rate of the lean NOx catalyst.
More particularly, the NOx purification rate of the lean NOx catalyst is significantly affected by not only the amount of HC supplied to the catalyst but also the composition of the HC and the catalyst temperature (or exhaust gas temperature). For example, when the exhaust gas temperature is low, small molecular weight HC (the carbon number of an HC molecule is equal to or less than 5) is effective to purify NOx, and when the exhaust gas temperature is high, large molecular weight HC (the carbon number of an HC molecule is equal to or more than 6) is effective to purify NOx. Regarding the size of an HC molecule, a small molecular weight HC is effective at low temperatures because the small molecular weight HC can be easily partial-oxidized at low temperatures to generate radicals capable of reducing NOx, while a large molecular weight HC is effective at high temperatures because the large molecular weight HC is cracked at high temperatures into small molecular weight HCs which in turn are partially oxidized to generate radicals. Regarding the catalyst temperature, the lean NOx catalyst has a temperature window where it can show a high NOx purification rate; the peak NOx purification rate is in the range of 400.degree.-500.degree. C.
Further, the composition of HC included in exhaust gas changes with a change of the engine speed (NE) and a change of the EGR rate. For example, when NE increases, the amount of large molecular weight HC in the exhaust gas decrease. When the EGR rate is increased, a large amount of large molecular weight HC is produced at high engine speeds, though the amount of large molecular weight HC does not change at low engine speeds. Further, in the lean burn engine, when the EGR rate is increased, the lean limit of the engine shifts toward the rich side. As a result, the air-fuel ratio has to be changed to a richer region of a lean air-fuel ratio range, and the exhaust gas temperature rises.
Thus, when the EGR rate is determined based on the HC concentration only, as in the prior art, the EGR is not utilized most effectively to purify NOx, because a change in the HC component due to the EGR and a change in the exhaust gas temperature due to the EGR are not taken into account.